This invention relates to digital data communication, and more particularly to methods and apparatus for automatically adjusting the circuitry involved in such communication to compensate for losses in a digital data signal transmitted from driver circuitry to receiver circuitry.
Different signal transmission media tend to have different signal transmission characteristics. For example, a cable may have a different transmission characteristic than a printed circuit board backplane. In addition, each instance of any given type of transmission medium may have somewhat different characteristics, within a range that is typical for that type of transmission medium. It is also possible for a transmission medium to change over time or as a result of other environmental factors.
Among the characteristics that can adversely affect the performance of a transmission medium are attenuation and phase shift. It is common for the amount of attenuation and phase shift to be frequency-dependent. Typically, both attenuation and phase shift tend to increase with increasing frequency. For convenience herein, attenuation, phase shift, and other forms of signal degradation are sometimes referred to generically as “losses.”
In order to have satisfactory transmission of a digital data signal, especially at high data rates or high frequencies, it may be necessary to compensate for losses in the signal being transmitted. Moreover, because such losses can vary from instance to instance and from time to time, it can be desirable for such compensation to be at least partly automatic or adaptive. A term that is often used for such compensation is equalization. The term pre-emphasis is also sometimes used for compensation or equalization that is performed at the driver, i.e., anticipating losses that will occur and compensating for them by modifying the signal before it is transmitted. When the term pre-emphasis is used, equalization may then be used as the term for compensation performed at the receiver. Although equalization is described as being performed primarily at the receiver, it should be recognized that some aspects of this invention may also be applied to equalization (pre-emphasis) performed at the driver.
Commonly, equalization is provided by non-programmable circuits that have a fixed amount of gain for a particular frequency. Such circuits may be identified and used with transmission media having suitable attenuation characteristics. For example, an equalization circuit providing 2 dB of gain to a 100 MHz signal may be used with a backplane that attenuates 100 MHz signals by 2 dB.
Programmable circuitry such as programmable logic device (“PLD”) circuitry has capabilities that can be useful in supporting adaptive equalization. For example, a PLD or PLD circuitry may be one of the components involved in transmitting or receiving a signal needing adaptive equalization, or such circuitry may be used for controlling certain aspects of the circuitry that transmits and/or receives such a signal. Such programmable circuitry (e.g., PLD circuitry) can be especially useful in implementations of this invention because programmability aids in providing different parameters and/or procedures for addressing different transmission loss characteristics that may be encountered.